From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335

Fall 1998, No. 6


The objective of soil sampling should be to identify fertilizer and lime needs for different areas within fields that may respond differently to management inputs. If the fertility differences are great enough, different rates of lime and/or fertilizer can be applied with conventional single-bin, single product applicators or multi-bin, multiple-product, variable-rate applicators.

Whether GPS (Global Positioning System) is used, or "dead reckoning" of actual distances and directions between sample locations from a permanent reference point, the key is to be able to go back to the same sample points over time. This will provide the greatest opportunity to minimize the impact of spatial variability when comparing sample results over time. Changes in soil fertility as a result of crop nutrient removal, differences in soil buffering, and phosphorus and potassium fixation effects can be more accurately evaluated. For many involved in intensive grid sampling by GPS, this year or next may be the 3rd or 4th year since the original samples were collected. Some suggest that every grid cell or point in the first sampling program will not need to be sampled again, but only representative points. At the very least, crop yield variability and variations in soil texture and soil moisture should be considered when developing the second-stage or resampling strategy, if a grower is not committed to a complete resampling effort.

A grower who is just beginning an intensive soil sampling program may elect to start with a systematic unaligned grid approach where individual soil cores are collected in a radius around each sampling point. Several software programs permit flexibility in establishing the sample points. If a grower has considerable knowledge of yield patterns in a field, has historic and recent aerial photographs, or has knowledge of drainage patterns within the field, these should be studied before the grid sampling pattern is arbitrarily imposed. More and more often, fertilizer dealers, crop consultants, and farmers are observing that soil moisture holding capacity and drainage are the factors most affecting yield variability from year to year. Soil survey maps may help to identify potentially different management zones in many fields where there are differences in depth to a fragipan or clay pan, or depth to strong subsoil acidity, that could affect crop rooting depth and soil moisture availability.

Observant farmers and crop advisers recognize crop stress patterns as a consequence of weather-related effects. If grid sampling for the first time, or resampling, such patterns can be used to plan a soil sampling scheme that better matches productivity differences within the field. If there are significant differences in slope, aspect, and moisture drainage within a field, the soil sampling plan should take these differences into consideration.

Soil fertility interacts with many other factors to influence crop yields: Soil moisture, soil temperature, surface and internal drainage, compaction, nematodes, root diseases and foliar diseases, insects, tillage, past lime and/or fertilizer application patterns, weed populations, and herbicide residues and their interaction with soil organic matter…just to name a few. Field knowledge of these other factors can improve the value of soil test results in developing nutrient management plans. If specific patterns of influence are known for these other factors, "zonal" sampling or directed sampling points can be assigned, as separate sampling locations or as part of the overall sampling scheme. As research identifies ways to define productivity regions within fields and opportunities arise for variable-rate nitrogen management, directed or zonal sampling will become more and more important. More accurate fertility management maps may be developed, improved returns from fertility management may be realized, and improved environmental stewardship can be achieved.

Take advantage of your agronomic observations and the technology tools to help remove soil fertility as a limiting factor in profitable crop production. Use common sense to make sense of your intensive soil sampling and nutrient management program.


For more information, contact Dr. Cliff S. Snyder, Midsouth Director, PPI, P.O. Drawer 2440, Conway, AR 72033-2440. Phone (501) 336-8110. E-mail:
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